The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present invention is related in general to fluid compositions and methods for servicing subterranean wells. Particularly, the invention relates to the use of fluids containing fibers to separate two wellbore-service fluids as they travel through a tubular body. Such fluids are also referred to as “fiber-laden fluids.”
Well-servicing operations frequently involve the pumping and placement of fluids in the wellbore. In many cases, different fluids are pumped through tubulars in a sequence. Perhaps the most common example is primary well cementing, during which the fluid sequence may involve drilling fluid, followed by a spacer fluid, and then followed by one or more cement-slurry formulations. It is desirable to minimize or prevent commingling of the fluids for two principal reasons. First, mixing of fluids at their interfaces reduces their useful volumes. Second, the fluids may be incompatible, leading to unwanted rheological consequences such as gelation. The fluid mixture may viscosify, increasing the friction pressure during pumping. Or, one fluid may penetrate the interface of the other, forming channels. Maintaining the integrity of the cement slurry is of great importance, because contamination may significantly alter the rheological performance, preventing proper placement in the annulus. In addition, the ability of the set cement to provide zonal isolation may be compromised.
Several techniques are available to isolate or minimize the commingling of fluids as they travel through tubulars. The methods fall into two principal categories: hydrodynamic or mechanical.
Hydrodynamic techniques involve adjusting the rheological properties of the fluids and the pumping rate. The aim is to achieve a flow regime that will minimize commingling. If both fluids are in turbulent flow during placement, mixing at the interface will be minimized. However, if the fluids are incompatible, even minimal mixing may increase the viscosity at the interface, causing the fluid in the mixing zone to assume laminar-flow, and exacerbating further mixing and contamination. If turbulent flow is not feasible, the fluids may be designed to have significantly different gel strengths, and the pumping rate may be reduced so that both fluids are in plug flow during placement. The flat interface between the fluids that is characteristic of plug flow, coupled with the gel-strength differential, discourages mixing. In many cases either technique may not be feasible because formulating the fluids to achieve a desired flow regime may negatively impact other critical aspects of fluid performance. In addition, the necessary pumping rates may not be optimal with respect to other well engineering parameters.
Mechanical methods are more frequently employed to achieve isolation between fluids, the most common being wiper plugs and foam balls. These devices effectively prevent mixing during displacement; however, special hardware at the wellhead is required to deploy them, and additional hardware must be installed at the exit of the conduit to hold them in place after arrival. In addition, working with these devices is time consuming. Finally, the tubular string often consists of sections with multiple diameters, and complex assemblies are required to ensure that the mechanical devices are adapted to the current diameter.
It therefore remains desirable to provide means to isolate two fluids and prevent mixing, without the need to adjust the fluids' rheological properties, without the need to provide special hardware at the wellsite, and without regard to tubular-diameter changes.